The first external fixator was developed in 1843 for reducing and maintaining patellar fractures. Since then a large number of different fixators have been invented for splinting various bone fractures. Virtually all of these fixators have some features in common. In particular, they rely on transcutaneous pins or screws secured in the bone on either side of the fracture site. An external mechanism is attached to the pins and allows their relative positions to be adjusted. This enables the surgeon to reestablish alignment of the bone pieces at the fracture site. Once the bone is properly set, the articulations in the fixator are locked in place to maintain the chosen alignment.
The principle variations among the many fixator designs are the number of degrees of freedom provided and the relative independence of each articulation, both mechanical and geometric. The first fixator, for instance, was adjustable only in length and squeezed the fracture together by gripping opposed ends of the patella. Fixators designed to repair central fractures of the long bones typically have relatively few articulations or degrees of freedom. In contrast, fixators adapted to treat fractures of bones in the neighborhood of joints must provide many more degrees of freedom. Where there is not room to place the pins in the fractured bone between the fracture and the joint, the additional degrees of freedom are necessary because alignment must be established using pins placed in a bone on the far side of the joint from the fracture. For treatment of fractures near joints such as the wrist, which can rotate, flex and abduct, the fixator should offer some equivalent adjustment to accommodate the flexibility of the skeletal joint to allow the surgeon to establish the proper fracture alignment using forces transmitted through the joint.
Modem fixators tend to provide a large number of articulations of varying kinds. Probably the most common articulation is the ball joint. A ball joint provides one rotational and two pivotal degrees of freedom. A single set screw or other locking mechanism can fix all three degrees of freedom simultaneously. The disadvantage of this type of articulation is that it is not possible to loosen the joint for motion in only one of the degrees of freedom without loosening it to move in other degrees of freedom. Thus, a surgeon cannot loosen the ball joint slightly to pivot it a small amount in one direction without the possibility of introducing changes affecting the other pivot and rotation settings.
In order to overcome this limitation, some fixators eliminate ball joints and rely instead on a combination of independent articulations to provide the necessary flexibility. The benefit of such a system is that each degree of freedom is mechanically independent of every other degree of freedom. A surgeon can, thereby, adjust the position of a single articulation in the fixator without affecting the settings of other articulations. Unfortunately, a given geometric readjustment of the bone ends at the fracture site may not correspond to an adjustment of any single articulation. Proper readjustment may require the surgeon to adjust several separate articulations, eliminating much of the benefit of independent articulations. Moreover, movement of one articulation may change some alignment of the bone ends previously established by another articulation.
With single degree of freedom articulations, such as simple pivots or slides, there are two basic adjustment techniques--gear driven and free. Free articulations are typically freely adjustable until some type of lock is applied to secure the articulation at a selected setting. When the lock is loosened the articulation is relatively free to move as the surgeon applies force to the joined members. Gear driven articulations, in contrast, move under the control of some adjustment mechanism which provides mechanical advantage, such as a worm gear and rack or similar structure. Turning the worm gear causes the articulation to move incrementally in accord with the rotation of the worm gear. This latter type of articulation generally provides the surgeon greater precision and control when making fine adjustments, but hinders rapid gross corrections. It is possible to provide an articulation having both properties, however, in order to allow free motion of the articulation, the mechanical advantage provided by the gear reduction must be rather minimal. This would reduce the precision of the adjustment and negate the very purpose for which a gear drive would be used in the first place.
Most fixators also include some type of extensible/contractible articulation to permit the longitudinal spacing between the pins on opposite sides of the fracture to be controlled. This type of translational freedom can be used to accommodate individuals of varying size, as well as to distract the fracture, if necessary. In addition, for general purpose fixators which are not designed for a specific fracture, translational degrees of freedom can be used to create whatever spacing is required on either side of the fracture to allow for proper pin placement.
Fixators may be either general purpose or fracture specific. General purpose fixators are designed with considerable flexibility to accommodate many different types of fractures whereas fixators intended for use on a specific type of fracture typically have fewer degrees of freedom. In addition, the articulations provided are usually tailored to correct for specific fracture displacements. Likewise, for fractures too close to a joint to permit pin placement on both sides of the fracture, the articulations are adapted to compensate for varying joint position. Articulations corresponding to joint movements may also be used to set the joint in a comfortable position, as well as align the ends of the bone at the fracture site.
One of the more common fractures requiring a fixator for proper treatment is a fracture of the distal radius, or Colles fracture. This type of fracture usually results from a fall on an outstretched hand. The fracture line is usually quite close to the distal head of the radius, and, because of the lack of space and the number of tendons and nerves in the area, it is not possible to mount pins in the radius on the distal side of the fracture. Therefore, such fractures are reduced using a pair of pins set in the metacarpal bone and a pair of pins set in the radius on the proximal side of the fracture. In order to avoid damage to tendons and nerves, the radial pins are usually set in the third quarter of the radius, i.e. the proximal half of the distal half of the radius. Since the pins are set on opposite side of the wrist joint, the fixator must be sufficiently articulated to reduce the fracture using forces transmitted through the wrist joint.
The wrist joint permits the hand to move in three degrees of freedom relative to the forearm. First, the hand can move in supination and pronation, i.e. the rotation about the longitudinal axis of the forearm. Second, the hand can move in adduction and abduction, i.e. pivoting about an axis perpendicular to the plane of the palm. The last type of mobility of the hand is flexion and extension, which is the pivotal motion about an axis in the plane of the palm and perpendicular to the longitudinal axis of the forearm.
An example of a fixator designed for the treatment of Colles fractures is disclosed in U.S. Pat. No. 4,992,896 to Agee et al. (Agee '896). In operation, the device is mounted on two pairs of pins as described above. The first pair of pins is carried by a metacarpal bar mounted in a trolley so that it can pivot about an axis parallel to the axes of the pins, as well as translate toward and away from the trolley along the same axis. The translational position of the bar relative to the trolley is controlled by a gear drive and the pivotal motion is a free articulation with a lock.
The trolley is movably mounted to an elongate distal element and is positioned along distal element using a rack and co-acting worm gear. The distal member is joined to a second element through a pivot joint having an axis that forms an acute angle with the longitudinal axis of the distal element. The second element is in turn coupled to a third element by a pivot joint. The second pair of pins are mounted in the third element, and both the pivotal axes connecting the second element to adjacent elements intersect the distal pin of the second pair. The pivot axis between the second and third elements is specifically coaxial with the axis of distal pin. Both pivotal joints are gear driven using worm gear/rack mechanisms.
Of the five degrees of freedom provided by the Agee '896 fixator, four are gear driven articulations, rather than free moving. The large fraction of gear driven articulations in Agee makes the fixator relatively easy to fine tune once it is place, but also make it more difficult to initially install. The first step in the process of installing the fixator on the patient is placing the pairs of pins in the metacarpal and radius. After the location of the pins is established and they are installed in the bones, the surgeon installs the fixator over the free ends of the pins. Because most of the articulations in the Agee '896 fixator are gear driven, the surgeon must carefully preset each fixator articulation to match the pin placement. If the articulations were free moving, the surgeon could simply loosen the locks and flex and move the articulations as necessary to fit the pin placement.
An additional disadvantage of the Agee '896 fixator is the requirement that the axes of the pins all be substantially parallel. This is necessary because the Agee '896 patent does not have an articulation to fully accommodate the pin axes in the metacarpal being misaligned with the pin axes in the radius. The pin misalignment could be in either of two forms--abduction or supination. Since the metacarpal bar pivots freely about an axis generally parallel with flexion pivot axis of the wrist, misalignment in this direction in not critical. A slight misalignment in supination can be compensated for by using the pivot articulation between the distal and second elements. However, because the two pivot articulations between the second element and adjacent elements have only a small range of motions, approximately 15-20 degrees on either side of neutral, if the pivots must be adjusted to compensate for supination pin misalignment, there may not be sufficient travel left to properly reduce the fracture. There is no adjustment whatsoever for accommodating abduction misalignment.
Another deficit resulting from the lack of adequate supination and abduction flexibility in the Agee '896 fixator is the inability to set the wrist joint to a comfortable resting position in some cases. The resting position of a relaxed wrist is about 14 degrees extended and about 15 degrees abducted. While the Agee '896 fixator provides adequate flexion range, it does not provide any adjustment for abduction, thus forcing the metacarpal into parallel alignment with the radius--some 15 degrees away from the resting position.
Another drawback to the Agee '896 fixator is that the pivot axis of the flexion articulation of the metacarpal bar does not correspond to the pivot axis of the flexion of the wrist. Thus adjusting the flexion using the metacarpal bar pivot will disrupt alignment of the bone ends at the fracture site.
In addition to the requisite physical characteristics of a fixator, it is important to consider the psychological impact of the fixator on the patient. The sight of pins passing through the wearer's skin can be distressing to the wearer, as well as other people who may come into contact with the wearer. This may be particularly true during meals and in public. It is therefore desirable to mitigate the deleterious psychological impact of wearing a fixator, to whatever extent possible.
It is therefore an object of the present invention to provide a fixator for use on fractures of the distal radius or wrist.
It is another object of the present invention to provide a fixator for use on fractures of the distal radius that is articulated to allow adjustment of each of the three degrees of freedom of the wrist.
It is an additional object of the present invention to provide a fixator, for use on fractures of the distal radius, that provides a sufficient range of mobility to accommodate wrist flexibility and imprecise pin placement and still have enough travel left to reduce the fracture.
It is yet another object of the present invention to provide a fixator for use on fractures of the distal radius with enough free articulations to facilitate easy mounting on the support pins after they are installed in the radius and metacarpal bones.
An additional object of the present invention is to provide a fixator for use on fractures of the distal radius that allows the surgeon to achieve accurate and rapid reduction of the fracture.
One more object of the present invention is to provide a fixator including an enveloping cover to make the fixator more cosmetically acceptable.
These and other objects and advantages will be more clearly understood from a consideration of the accompanying drawings and the following description of the preferred embodiment.